What i want to do is: selecting points, which are transformed to circles using their seen_count as radius, intersect with bounding box. Is it possible? If so, would it be an expensive query? Thanks in advance.

Wrap the table.geom in ST_Buffer() using the seen_count column as buffer distance, like so: SELECT id, ST_AsGeoJSON(the_geom) FROM table WHERE ST_Contains(ST_MakeEnvelope({0}, {1}, {2}, {3}, 4326), ST_Buffer(table.the_geom, table.seen_count)). I guess you realize that ST_Contains will return the ids only of points where the buffered circle it totally within the bounding box.
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MichaJun 23 '13 at 11:41

Thank you for showing me a starting point. I found the answer!
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onatmJun 23 '13 at 19:48